1. Field of the Invention
The present invention relates to methods and apparatus for echo cancellation in communications systems employing multi-carrier data modulation.
2. Discussion of the Related Art
Modern telecommunications systems typically employ duplex data transmission in which data is simultaneously transmitted in opposite directions between transceivers of the telecommunications system using data channels having overlapping frequency bands. Any imperfections in the transceivers means that, in a given transceiver, data signals transmitted by the transceiver may be permitted to leak into, and interfere with, signals received by the transceiver thereby degrading the quality of those received signals.
For example, in a common telephone handset a transmit signal representing voice data from a microphone is typically input to a hybrid transformer for subsequent transmission to a remote telephone handset. The transformer will typically allow most of the transmit signal to be transmitted to the remote telephone via a duplex transmission line. However, imperfections in the hybrid transformer will often cause some of the transmit signal to leak through the transformer, into the receiver of the handset and subsequently into the earpiece of the handset thereby interfering with the receive signal. As a result, while speaking into the handset, a person may well hear an “echo” of their own voice in the handset earpiece.
This is just one manifestation of signal “echo” in data transceivers, and in telephones it is an irritation at the very least. However, in other data transmission systems, such as modems, echo signals may present a major obstacle to efficient duplex data transmission causing serious corruption of received data.
Echo cancellation techniques have been developed in an attempt to mitigate the problems caused by echo signals. Typically, echo cancellation involves emulating an echo signal and subsequently subtracting that emulated signal from received signals in which an echo signal (which the emulated echo signal is intended to emulate) may exist. Thus, if echo emulation is exact, subtraction of the emulated echo from the received signal will result in a fully echo-free or echo-cancelled received signal.
The transceiver of a duplex telecommunications system may be regarded as providing an “echo path” from the transmitter part of the transceiver to the receiver part via e.g. the hybrid transformer thereof. By considering the “echo path” to define a filter with a frequency response or transfer function relating its input (i.e. the leaked transmit signal) to its output (i.e. the echo signal) one can attempt to emulate the echo signal by building a filter which emulates the frequency response or transfer function of the echo path. By passing a portion of a given transmit signal through the emulating filter an echo emulation may be produced substantially contemporaneously with the creation of the echo being emulated such that both derive from the same transmit signal.
Echo cancellers typically employ this technique, and their efficacy is sensitively dependent upon their ability to efficiently and accurately generate filter parameters (“echo parameters”) for the echo-emulating filter. The generating of echo parameters is usually performed solely in the time-domain by emulating e.g. the echo path impulse response, with echo signal emulation subsequently employing Finite Impulse Response (FIR) filters to apply those echo parameters.
However, echo cancellation in purely the time-domain is computationally complex and relatively inefficient with echo parameter calculations requiring computationally expensive convolution calculations. Consequently, to avoid some of the deficiencies inherent in full time-domain echo emulation, techniques have been developed in which some echo parameter calculation is performed in the frequency domain with the consequence that computational complexity is reduced (note that a convolution in the time-domain is equivalent to a relatively simple multiplication in the frequency-domain). However, these techniques still require at least some echo parameter generation in the time-domain. In frequency-domain echo cancellation, one may obtain echo parameters merely by sampling the spectrum of the echo path rather than having to sample the response of the echo path which is relatively more computationally intensive.